1. Field of the Invention
This invention relates to narrow-band RC active bandpass filters and in particular to narrow-band filters in which the absolute bandwidth is substantially constant and is substantially independent of the center frequency.
2. Description of the Prior Art
In keeping with the size reduction in components, it has been necessary to devise new techniques to obtain filter networks of comparable reduced size. Thus, inductor simulation has been refined and the use of RC networks with operational amplifiers to obtain bandpass, low-pass, and bandstop filters is well known. A problem that was created by the use of miniature and integrated circuits was that of obtaining high resistance values in excess of the values of resistance available in such circuits. One way in which this problem was solved was to switch resistors in and out of the circuit. In this way, the effective resistance is increased well beyond the actual values available and in direct proportion to the ratio of the off-period to the on-period (resistor connected in the circuit).
Using this technique, filters reduced in size but which approximate the characteristics of the prior conventional large size filters were derived as well as reduced size filters which permit the scaling of transfer functions, N-path multiplex filters and digital filters. However, none of the reduced size filters can be used as a narrow-bandpass filter in which the center frequency may be changed by digital programming and in which the absolute bandwidth remains substantially constant.